EP2053018A1 - "Borohydride foam; production method; hydrogen generation method using borohydride foam. - Google Patents

Abstract

Solid foam (I) of at least one alkali metal borohydride or alkaline earth, is claimed. Independent claims are included for: (1) a method of producing (I), comprising solubilizing a powder of at least alkali metal borohydride or alkaline earth in a solvent with agitation to obtain a solution, where the solvent has preferably a low boiling point; bubbling an inert gas in the solution maintained under agitation; evaporating the solvent, without stopping or agitating or bubbling to obtain a pasty material; and heating the pasty material to dry under reduced pressure to obtain (I); and (2) a process of hydrogen generation by hydrolysis of the alkali metal borohydride or alkaline earth, where the borohydride is a foam.

Description

• un nouveau matériau : une mousse solide d'au moins un borohydrure de métal alcalin ou alcalino-terreux ; ladite mousse pouvant aussi renfermer une matière catalytique ;
• un procédé de production de ce matériau ;
• l'utilisation de ce matériau pour générer de l'hydrogène par hydrolyse ; en d'autres termes, un procédé de génération d'hydrogène par hydrolyse dudit matériau.

The present invention relates to:

• a new material: a solid foam of at least one alkali or alkaline earth metal borohydride; said foam may also contain a catalytic material;
• a method of producing this material;
• the use of this material to generate hydrogen by hydrolysis; in other words, a process for generating hydrogen by hydrolyzing said material.

This new material finds particular application as a fuel for fuel cells. More particularly, in the context of this application, the present invention relates to fuel cells for the power supply of portable electrical or electronic devices, that is to say devices requiring low electrical power. The present invention may, however, also find application for supplying hydrogen fuel cells of higher powers.

One of the known hydrogen production methods consists of hydrolyzing borohydrides, in solid or liquid form, such as borohydride or sodium tetrahydroborate NaBH ₄ , dissolved and brought into contact with a solid state catalytic material. The hydrolysis reaction of such a borohydride takes place under certain conditions according to the following reaction scheme:

M (BH ₄ ) n + 2nH ₂ O ---> M [B (OH) ₄ ] n + 4nH ₂

where M is an alkaline or alkaline earth element and n is a positive integer equal to the number of valence electrons of the element M. This element M may therefore, for example, be sodium (Na), in which case the number n is equal to 1, which leads to the following reaction scheme:

NaBH ₄ + 2H ₂ O ---> NaB (OH) ₄ + 4H ₂ .

The element M may also consist of potassium (K), lithium (Li) or another suitable element.

According to this reaction scheme, sodium borohydride, commonly used, generates four moles of hydrogen by reacting with two moles of water.

In general, an aqueous solution of sodium borohydride is used and, to facilitate the reaction, catalysts. This aqueous solution is brought into contact with a catalytic material; typically, it passes on a catalytic bed, in order to generate hydrogen. Flow control of the borohydride solution controls the reaction. Nevertheless, it has been found that such a mode of hydrogen generation has a rather low yield; the yield being defined here as the ratio of the mass of hydrogen generated to the fuel mass used. Indeed, the use of a borohydride solution has a major disadvantage for the application to fuel cells, in particular for portable devices: the solubility in water of sodium borohydride and its reaction product is relatively weak.

Thus, about 3.8 moles of water are required to solubilize one mole of sodium borohydride (NaBH ₄ ) and 13.3 moles of water to solubilize one mole of the reaction product (NaBO ₂ ). The amount of water to be used to obtain either a solution of sodium borohydride or a solution in which the reaction product is soluble is therefore much greater than the amount of water required for the simple hydrolysis reaction.

As a result, the actual hydrogen generation efficiency is lower than the theoretical yield (10.8%). It would thus be about 7% if the solution solubilized only sodium borohydride (NaBH ₄ ), but it is of the order of 3% only because the solution solubilizes said sodium borohydride (NaBH ₄ ) and the reaction product. (NaBO ₂ ). There is therefore currently no hydrogen generator based sodium borohydride solution, capable of working with yields greater than 4%.

In addition, it should be noted that, in general, the borohydride solutions are not stable over time: they hydrolyze by generating hydrogen (H ₂ ), which proves troublesome for storage.

In order to approximate the theoretical yield and to increase the energy density of the fuel, it is therefore preferable to use only the amount of stoichiometric water required for the hydrogen generation reaction. For this, it has been envisaged to use borohydrides in solid form, in particular in the divided state, that is to say in powder form. The hydrogen is then generated by addition of solid borohydride to an aqueous solution, preferably containing a catalytic material.

However, it is difficult to control such a reaction and hence the hydrogen flow rate generated. Indeed, this type of heterogeneous reaction, that is to say involving reagents in the liquid state and in the solid state, is difficult to control because it involves diffusion mechanisms, in this case the diffusion of the water towards the borohydride. However, such diffusion mechanisms strongly depend on the porosity of the reactive medium, which itself varies during the progress of the reaction, due to the formation of more or less compact by-products.

Work has been done to try to improve the diffusion of water in catalyzed borohydride substrates. Materials resulting from this work make it possible to better control the kinetics of reaction, and therefore the flow rate of hydrogen generated, and in addition to limit the formation of a passivating layer. These are borohydrides in the form of powders whose grains are covered, in whole or in part, with catalytic material, or contain, or are intimately mixed with, particles of catalytic material.

The patent US6,746,496 thus describes micro-dispersed powders of borohydride particles containing a reaction catalyst. The patent application FR 2,892,111 it describes a borohydride powder whose grains are covered in whole or in part by particles of catalytic material. Said coated grains are obtained from, on the one hand, a suspension of powder grains ("virgin") and, on the other hand, of a solution containing in solution a salt or alloy or oxide of at least one metal noble or not noble. Said powder grains "virgin" have a specific surface between 0.5 and 100 m ² .g ^-1 , preferably between 1 and 10 m ² .g ^-1 . The management and the impact of the granulometry and the specific surface area of the grains (of a powder) are in no way comparable to the management and the incidence of the porosity and the specific surface area of a foam (see below). A reaction, when said suspension and solution are contacted at a suitable temperature, by consuming powder (NaBH ₄ , for example), reduces said salt, alloy or oxide (ruthenium acetylacetonate, for example) and causes the precipitation of particles of said at least one metal at the surface of the powder grains. Such a method therefore consumes powder and certainly can not guarantee the size of the particles of catalytic material deposited on the surface, surface only, grains of powder. The patent application WO 2005/005311 deals with obtaining tablets or pellets by compression of catalyzed borohydride powders (intimate mixtures obtained by evaporation of a solvent). The same patent application WO 2005/005311 also presents materials obtained by deposition of borohydride (optionally catalyzed) on supports (porous material type or metal or organic foam).

Although it is recognized that the above materials lead to a better controlled kinetics of hydrolysis, the latter is still insufficient, in particular because of the formation of a passivating layer limiting the diffusion of water in the substrate.

• que leur porosité doit être suffisante pour recevoir un volume important d'hydrogène (ces matériaux sont en effet utilisés pour stocker l'hydrogène généré) ou pour permettre à l'hydrogène de passer à leur travers. L'homme du métier comprend qu'ils doivent donc présenter une surface spécifique importante (i.e. de "petits" pores) ;
• qu'ils peuvent être obtenus soit par action d'un gaz, avec variation de pression, soit par action d'un agent porogène susceptible de subir une réaction de décomposition chimique, thermique ou photolytique.

The patent application US 2007/020172 discloses a device for generating hydrogen in which two reactants, held in contact through a porous wall, react to generate hydrogen. The reaction in question - first solid reagent containing hydrogen (sodium borohydride, for example) / second liquid reagent (eg water) - is a surface developed reaction of the solid material. Said solid material is capable of intervening in the form of powders and granules, advantageously in the form of compressed rolls or bars. It is also capable of acting as foams, cell solids or the like. These porous materials are not characterized. It is indicated :

• that their porosity must be sufficient to receive a large volume of hydrogen (these materials are indeed used to store the hydrogen generated) or to allow hydrogen to pass through them. The skilled person understands that they must therefore have a high specific surface (i e "small"pores..);
• that they can be obtained either by the action of a gas, with variation of pressure, or by action of a porogenic agent capable of undergoing a chemical, thermal or photolytic decomposition reaction.

Those skilled in the art are in fact always looking for a solid material, such as a borohydride, advantageously catalyzed, likely to lead to accelerated kinetics of hydrogen generation by hydrolysis.

In such a context, the Applicant is currently proposing a new material capable of generating hydrogen by hydrolysis of at least one borohydride, making it possible to better control the kinetics of reaction, therefore the flow of hydrogen generated, and further to limit the formation of a passivating layer.

According to its first object, the invention relates to a material consisting of a foam of at least one borohydride of alkali metal or alkaline earth metal; said foam having a specific surface area of between 1.5 m ² .g ^-1 and 10 m ² .g ^-1 .

The element (s) constituting (s) said material is (are) not per se original (aux): at least one borohydride of an alkali metal or alkaline earth metal, usually a borohydride of a alkali metal or borohydride of an alkaline earth metal; mixtures of at least two alkaline borohydrides, at least two alkaline earth borohydrides, or even at least one alkaline borohydride and at least one alkaline earth borohydride (mixed mixtures) are however not excluded. What is original is the shape of the material. Typically, the said constituent element (s) has been conditioned into a solid foam having a specific surface as indicated above.

The material of the invention is new in that it is a solid foam having such a specific surface. This concept of solid foam is familiar to those skilled in the art, in contexts different from that of the invention. The expression "solid foam" in said contexts, and therefore in the present description and the appended claims, means a homogeneous solid material with a high porosity and a specific surface area. In the context of the present invention, the specific surface area of said foam is greater than 1.0 m ² .g ^-1 (it is a foam): it is between 1.5 m ² .g ^-1 and 10 m ² .g ^-1 . It is an open porosity, free, although it is not excluded that some pores are closed.

The concept of specific surface, more particularly used to characterize a material, in the form of powder or foam, is familiar to those skilled in the art. It is classically determined by the BET method (see below).

The specific surface area values set forth above for the foams of the invention are sufficiently high to characterize foams but are, in any case, not very high (for foams). Thus, the foams of the invention contain sufficient large pores (in view of said specific surface values) for allow easy penetration of the hydrolysis reagent within said foams (the hydrolysis reaction is developed in the volume of said foams), to optimize the borohydride reaction (s) / hydrolysis reagent within said foams. The foams of the invention actually offer an optimum in terms of reactive surface (overall) and accessibility to said reactive surface.

Advantageously, with reference to the constituent material of the solid foam of the invention, only one alkali or alkaline earth metal is involved and it is chosen from the group comprising sodium (Na), lithium (Li) and magnesium (Mg). ). NaBH ₄ sodium tetrahydroborate is the most advantageous because it is easy to produce and inexpensive.

The basic borohydride powders (raw materials useful for obtaining the foams of the invention: see the production process described below) generally have a specific surface area of between 0.05 and 0.1 m ² / g. (The skilled person is aware that such low values depend on the gas absorbed for the implementation of the measure). The foams of the present invention have a surface area more than 10 times greater than that of said basic borohydride powders (the measurement of specific surfaces of this order is much less dependent on the nature of the gas absorbed). However, it is not excluded that borohydride powders, which can be used to obtain foams of the invention, have a higher specific surface, and in particular a specific surface area of the order of that indicated for the foams of the invention. The coincidence of specific surface values (foams / powders) is only fortuitous.

The borohydride foam (s) of the invention may contain, in intimate mixture with (embedded in) the said borohydride (s) (within its constitutive element (s) ), catalytic material (particles of catalytic material) capable (s) to catalyze the reaction of hydrogen generation by hydrolysis of (or said) borohydride (s). This catalytic material, obviously intervenes with reference to the main outlet of the foams of the invention, presented in the introduction of this text. Incidentally, it is noted here that the foams of the invention, catalysed or not, may have other outlets. They can be used in other contexts.

The particle size of said catalytic material is as small as possible. The particles of catalytic material therefore generally consist of nanoparticles which have a median diameter of between 1 and 100 nm, advantageously between 3 and 5 nm. Such nanoparticles make it possible to obtain an intimate mixture with the metal borohydride. Their obtaining process is for example described in the following reference: Small C.; Pileni MP "Nanosize cobalt boride particles: control of the size and properties", Journal of Magnetism and Magnetic Materials vol. 166, p. 82-90, 1997 .

The catalysed foams of the invention generally contain from 0.5% to 30% by weight of catalytic material, advantageously at most 10%, very advantageously between 1 and 10%.

Said catalytic material, in intimate mixture with said borohydride (s), may consist of at least one metal selected from the group comprising platinum (Pt), ruthenium (Ru), palladium (Pd) ), cobalt (Co), nickel (Ni), iron (Fe), gold (Au), silver (Ag), manganese (Mn), rhenium (Re), rhodium (Rh) ), titanium (Ti), vanadium (V) and cerium (Ce), or by at least one alloy, or salt or oxide of at least one of said metals. These materials have the advantage of effectively catalyzing the hydrolysis reactions of borohydrides.

Advantageously, the salt in question is a chloride, a boride or an organometallic salt of at least one of the above metals, and for example cobalt boride or nickel boride, both of which are good catalysts.

The foams of the invention make it possible to generate hydrogen, by implementing a heterogeneous, solid-liquid, easily controllable reaction, whatever its degree of advancement.

According to its second object, the invention relates to a method for producing such foams.

• la solubilisation, avec agitation, d'une poudre d'au moins un borohydrure de métal alcalin ou alcalino-terreux dans un solvant, solvant préférentiellement à bas point d'ébullition (en référence à la mise en oeuvre de son évaporation ultérieure), pour obtenir une solution,
• l'éventuel ajout de particules de matière catalytique non soluble dans ledit solvant, avantageusement de granulométrie nanométrique, pour l'obtention d'une suspension,
• le barbotage d'un gaz inerte (type Ar, N₂) dans ladite solution ou suspension maintenue sous agitation,
• l'évaporation du solvant, sans arrêter ni l'agitation ni le barbotage, jusqu'à l'obtention d'un matériau pâteux, puis
• le chauffage sous pression réduite dudit matériau pâteux pour le sécher et obtenir une mousse solide dudit au moins un borohydrure.

The method comprises the following successive (basic) steps:

• the solubilization, with stirring, of a powder of at least one alkali metal or alkaline earth metal borohydride in a solvent, preferably a low-boiling solvent (with reference to the implementation of its subsequent evaporation), for get a solution,
• the possible addition of non-soluble catalytic material particles in said solvent, advantageously of nanometric particle size, to obtain a suspension,
• sparging an inert gas (type Ar, N ₂ ) in said solution or suspension kept under stirring,
• evaporation of the solvent, without stopping or stirring or bubbling, until a pasty material is obtained, and then
• heating under reduced pressure of said pasty material to dry and obtain a solid foam of said at least one borohydride.

It is conceivable that the properties (porosity, specific surface area) of the solid foam obtained depend on the conditions of implementation of the process for obtaining said foam and more particularly on the conditions for carrying out stirring, bubbling and blowing. Evaporation of the solvent. The management of these conditions, their optimization, in view of the desired result (specific surface values) falls within the knowledge of those skilled in the art.

In any case, it should be emphasized that agitation of the solution and bubbling are essential to obtain a foam.

The "block" of foam obtained can obviously be crushed to generate pieces (smaller) of foam.

In order to obtain a borohydride foam (s) which closes with catalytic material, the optional intermediate step specified above is carried out. Thus, the bubbling is carried out in a solution, in the absence of catalytic material, in a suspension, in the presence of catalytic material.

Those skilled in the art will be able to determine a solvent (solvent or solvent mixture) suitable for carrying out the above process, that is to say a solvent for the metal borohydride (s) retained. (s) solvent in which the catalytic material which intervenes possibly is insoluble. For example, mention is made of the liquid ammonia solvent for the sodium borohydride and metal chloride or boride pairs.

It is understood that the method of the invention has the advantage of being independent of the particle size of the borohydride (raw material), said borohydride being dissolved in the solution.

Regarding the particle size of the catalytic material, it has a certain importance and in the context of the process of the invention it can be optimized without difficulty. It is indeed perfectly controllable insofar as said catalytic material intervenes in the solid state (in the form of particles) and remains in suspension (in the same solid state, in the form of particles).

It is recalled here that the particles of catalytic material generally have a median diameter of between 1 and 100 nm, advantageously between 3 and 5 nm. Such nanoparticles make it possible to obtain an intimate mixture with the metal borohydride.

The present invention also relates to a process for generating hydrogen by hydrolysis of at least one foam of the invention (foam as described above and / or as produced by the method described above). Said method is advantageously used for the supply (in hydrogen) of a fuel cell. When the foam contains catalytic material, activation of the hydrolysis reaction requires no catalyst transport by the hydrolyzing phase.

The invention is hereinafter illustrated by the description of a particular mode of implementation of the method (example) as well as by the appended figures. The object of the invention is not however limited to this particular mode of implementation and other modes of implementation of the method are obviously possible.

The FIGS. 1A, 1B, 1C show the appearance, observed under a scanning electron microscope (at different magnifications), of a basic sodium borohydride powder serving as an ingredient (of raw material) for carrying out the process of the invention.

The FIGS. 2A, 2B, 2C show a foam, object of the invention, observed under a scanning electron microscope, at different magnifications.

Said figures are commented on in the following example.

Below is a practical example of implementation of the method - object of the invention - for manufacturing a foam - object of the invention - presently a sodium borohydride foam containing a catalytic material (cobalt boride ).

In this particular embodiment, in accordance with the process which is the subject of the invention, the sodium borohydride, of formula NaBH _4, is said to be dry because it is previously dehydrated at 150 ° C. under primary vacuum during 2 hours.

It is recalled that, in the context of the process of the invention, the particle size or specific surface of the borohydride (starting) does not matter since said borohydride to be dissolved. As a comparison with the foam object of the invention obtained at the end of the process (see FIGS. 2A, 2B and 2C ), the FIGS. 1A, 1B and 1C show images of scanning electron microscopy, at different magnifications, grains typical of basic sodium borohydride used as an ingredient. It is dense grains of about 50 to 200 microns, having a smooth surface. Thus, the sodium borohydride powder (NaBH ₄ ) used in the example described has a specific surface area of 0.08 m ² .g ^-1 .

The exemplified process also comprises the production of the particles of catalytic material, by reduction of a salt or an alloy or an oxide of at least one metal, according to methods described in the literature.

The particle size of the particles of catalytic material (cobalt boride, in the present example) is important in the context of the process of the invention, since these particles remain in suspension in the solvent (in the following process) until evaporation of said solvent. The smallest possible particle sizes are advantageously chosen, i.e. particles having a median diameter preferably less than 100 nm, very preferably less than 10 nm (see above).

In a typical preparation, 100 ml of ammonia gas is condensed in a reactor cooled to -35 ° C and topped with a dry ice trap. After liquefaction of the ammonia, 5 g of dry sodium borohydride (0.132 mmol) are rapidly introduced under vigorous stirring (by means of a 6-blade Rushton turbine, equipment well known to those skilled in the art), leading to the total solubilization of the borohydride. A mass of cobalt boride (small particle size) is added, corresponding to a level of 10% of the mass of borohydride introduced. After the introduction of the catalyst, an argon bubbling (flow rate of 20 l / min) is implemented by means of a capillary tube plunger in the solution and the temperature is gently brought down to ambient (1 ° C / 3min), which causes the evaporation of ammonia. Stirring is maintained throughout the duration (about 20 minutes) of evaporation of ammonia. When the mixture becomes pasty in the reactor, bubbling argon and stirring are stopped. The medium is then put under reduced pressure to a pressure of 10 ^-2 mbar and heated for 8 hours at 60 ° C. until a gray solid corresponding to the formed borohydride foam containing the catalytic material (foam which is an example of a foam according to the present invention). This foam must be stored away from moisture.

Its gray color testifies to the presence of the particles of the catalytic material, in this case cobalt boride. The elemental analysis indeed confirms the presence of 10% by weight of cobalt boride, in intimate mixture with (embedded in) sodium borohydride. Said elemental analysis ("ICP: Couple Plasma Induction") gives the following weight composition: 54.7% Na, 27% B, 7.3% Co and 11% H ₂ .

In addition, according to the main characteristic of the invention, the obtained material is in the form of an open-pore foam, as shown by the three images obtained by scanning electron microscopy, for different magnifications, shown on the FIGS. 2A, 2B and 2C ; the specific surface area (determined by the BET method, a method for determining the specific surface area based on the adsorption of gas, nitrogen or krypton, for example, at low temperature, in a monolayer on the surface of the sample to be analyzed) of the said foam being 1.61 m ² .g ^-1 (and its density is 1.2 g.cm ^-3 ). These values are characteristic of the foam state of the material.

It has been shown that the hydrogen production kinetics (expressed in μl of H ₂ / s) of the hydrolysis reaction carried out on the foam of the present example is at least 3.5 times faster than that recorded by hydrolysis implemented on a simple ground mixture of the ingredients of the example.

The material according to the present invention makes it possible to limit the formation of a passivating layer, that is to say a layer consisting of the by-products of the hydrolysis reaction. Indeed, the metaborates formed by contact between water, the catalyst and sodium tetrahydroborate are distributed on the surface of the foam, and thus, do not constitute a continuous and blocking layer.

Claims (11)

Solid foam of at least one alkali metal borohydride or alkaline earth metal, characterized in that its specific surface is between 1.5 m ² .g ^-1 and 10 m ² .g ^-1 . Foam according to Claim 1, characterized in that the said metal is chosen from the group comprising sodium (Na), lithium (Li) and magnesium (Mg). Foam according to claim 1 or 2, characterized in that it contains particles of catalytic material capable of catalyzing the hydrogen-generating reaction by hydrolysis of said at least one borohydride. Foam according to claim 3, characterized in that said particles have a median diameter of between 1 and 100 nm, advantageously between 3 and 5 nm. Foam according to claim 3 or 4, characterized in that said catalytic material represents between 0.5% and 30% by weight, preferably between 1 and 10% by weight, of the total weight of said foam. Foam according to any one of Claims 3 to 5, characterized in that the said catalytic material consists of at least one metal chosen from the group comprising platinum, ruthenium, palladium, cobalt, nickel, iron, gold, silver, manganese, rhenium, rhodium, titanium, vanadium and cerium, or by at least one alloy or salt or oxide of at least one of said metals. Foam according to Claim 6, characterized in that the said salt consists of cobalt boride or nickel boride. Process for the production of a solid foam of at least one alkali metal or alkaline earth metal borohydride according to any one of Claims 1 to 7, characterized in that it comprises: the solubilization, with stirring, of a powder of at least one borohydride, of alkali metal or alkaline earth metal, in a solvent, for obtaining a solution, the possible addition of particles of non-soluble catalytic material in said solvent, advantageously nanometric granulometry, for obtaining a suspension, the bubbling of an inert gas into said solution or suspension kept under stirring, the evaporation of the solvent, without stopping or stirring or bubbling, until a pasty material is obtained, and then - Heating under reduced pressure of said pasty material to dry and thereby obtain a solid foam. Process according to Claim 8, characterized in that the said particles of catalytic material have a median diameter of between 1 and 100 nm, advantageously of between 3 and 5 nm. A process for generating hydrogen by hydrolysis of at least one alkali or alkaline earth metal borohydride, characterized in that said borohydride is a foam according to any one of claims 1 to 7 and / or a foam produced by the process according to claim 8 or 9. Hydrogen generation process according to claim 10, characterized in that it is used for supplying a fuel cell.

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